1. Field of the Invention
The present invention relates to a user selection method for multiuser MIMO communication.
2. Description of the Related Art
In up-link wireless access of Next Generation Mobile Communication Systems, to improve cell throughput, multiuser Multiple Input Multiple Output (MIMO) communication (hereinafter, called “MU-MIMO”) may be expected to be used. In MU-MIMO, multiple terminals transmit data signals at the same frequency and a base station separates transmitted signals of multiple users which are viewed as MIMO signals. MU-MIMO may be viewed as a Space Division Multiple Access (SDMA) system in which a space channel forms a resource in addition to traditional resources of time and frequency, and a large, multiuser diversity effect can be provided by suitably selecting a pair of users performing simultaneous, multiple access.
FIG. 1 shows a configuration of an up-line portion in an MU-MIMO system. Here, each transmitter is supposed to have one transmission antenna, but it may be thought that each transmitter has multiple transmission antennas, and selects one for transmission, or transmits from the multiple transmission antennas.
Taking the number of simultaneous transmission users as M, user transmitters 201-1 to 201-M convert a data signal into an error-correcting code and perform digital modulation, respectively. Transmission antennas 202-1 to 202-M transmit a data signal of each user, respectively. Receiving antennas 203-1 to 203-M receive multiplexed data signal of each user. Receiver 203 separates the data signal of each user and corrects an error to decode the data signal. Further, receiver 203 selects a pair of users for data transmission according to each transmission time interval (TTI), based on measurements of a channel quality of each user. Received signal y in receiver 204 may be expressed by the following equation using transmission
symbol s(k)=[sJ1 sJ2 . . . sJM]T of selected (paired) users J1 to JM and
channel matrixH(k)=[hJ1 hJ2 . . . hJM]:y=H(k)s(k)+n  (1)Where, n is a noise vector.
There is a method based on a received signal to interference plus noise ratio (SINR) of each user as a user selection method of the related art. FIG. 2 shows a configuration of a device for user selection by a maximum CIR method. An example shown in FIG. 2 includes received SINR measurement means 101, user selection means 102 and completion determination means 103, and selects a user who has a large received SINR according to each TTI. Received SINR measurement means 101 measures the received SINR in up-link by using the pilot signal of each user (mainly the pilot signal for sounding periodically transmitted during non-transmission of data) the received SINR is generally used as an indicator representing channel quality, CQI (Channel Quality Indicator) in mobile communication systems. Thus, CQI may be used for the received SINR. User selection means 102 selects a user having a large received SINR. Taking a set of all users as A={1, 2, . . . , Nu}(Nu is the number of all the users), and the received SINR of a user j (1≦j≦Nu) as SINRj, user Jn to be selected in an n-th (1≦n≦M) MIMO multiplexed layer is selected by using the following equation.
                              J          n                =                  arg          ⁢                                          ⁢                                    max                              j                ∈                A                                      ⁢                          SINR              j                                                          (        2        )            
Further, selected user Jn is deleted from a set of users to be selected in the next MIMO multiplexed layer by using A=A−{Jn}.
Completion determination means 103 outputs user selection information after completing the selection of users based on the number M which represents the number of simultaneous transmission users, and moves to a process of a next user selection when n<M.
FIG. 3 shows situations of user selection by using a maximum CIR method when M=2. Users having the first and second largest received SINRs are selected according to each TTI, and their user transmitters are enabled to transmit data.
FIG. 4 shows a configuration of a device for user selection according to a proportional fairness (PF) method. The user selection method according to the maximum CIR method can achieve the multiuser diversity effect to the maximum extent by selecting a user having a good channel quality, but does not provide fair distribution (data transmission opportunity) of the selection frequency to each user. The PF method is a user selection method for making the data transmission opportunity of each user fair.
An example shown in FIG. 4 includes received SINR measurement means 701, weighted support data rate calculating means 704, user selection means 705, average support data rate updating means 706, and completion determination means 703, and this example shows a method for selecting a user having a large support data rate weighted by the reciprocal number of an average support data rate according to each TTI. Received SINR measurement means 701 measures a received SINR in up-link by using the pilot signal of each user. Weighted support data rate calculating means 704 calculates support data rate {tilde over (R)}j(SINRj) from the received SINR of each user, and calculates value {tilde over (R)}j(SINRj)/ Rj weighted by the reciprocal number of the average support data rate Ri.
To calculate {tilde over (R)}j(SINRj), there may be a method for referring to a table for SINR versus support data rate characteristics in an adaptive white Gaussian noise (AWGN) environment acquired in advance. That is, to indicate a SINR threshold value for realizing a required block error rate of modulation and coding scheme MCSi by Yi, a support data rate of MCSi is selected in Yi≦SINR<Yi+1. User selection means 105 selects a user having the support data rate as large as possible, weighted by the reciprocal number of an average support data rate. User Jn to be selected in an n-th MIMO multiplexed layer is selected by using the following equation.
                              J          n                =                  arg          ⁢                                          ⁢                                    max                              j                ∈                A                                      ⁢                                                                                R                    ~                                    j                                ⁡                                  (                                      SINR                    j                                    )                                                                              R                  _                                j                                                                        (        3        )            
Further, selected user Jn is deleted from a set of users to be selected in the next MIMO multiplexed layer. Average support data rate updating means 106 updates the average support data rate Rj by using the support data rate RJn actually transmitted.
                                          R            _                    j                =                  {                                                                                                                (                                              1                        -                                                  1                                                      t                            c                                                                                              )                                        ⁢                                                                  R                        _                                            j                                                        +                                                            1                                              t                        c                                                              ⁢                                          R                                              J                        n                                                                                                                                          (                                      j                    =                                          J                      n                                                        )                                                                                                                          (                                          1                      -                                              1                                                  t                          c                                                                                      )                                    ⁢                                                            R                      _                                        j                                                                                                (                                      j                    ≠                                          J                      n                                                        )                                                                                        (        4        )            where, tc is a mean time of Rj. Completion determination means 703 outputs user selection information after completing the selection of users by number M of simultaneous transmission users, and moves to a process of a next user selection when n<M.
The user selection method performed in each device shown in FIGS. 2 and 4, respectively, selects a user based on the received SINR of each user, but the user selection of MU-MIMO is affected not only by the received SINR, but by channel orthogonality of the pair of users.
FIG. 5 shows a configuration of a device for user selection by a whole search method in which the channel orthogonality is considered. The user selection by the device shown in FIG. 5 includes channel vector measurement means 811, user combination MIMO capacity calculating means 812, and user selection means 813, calculates MIMO capacity of a combination of all users, and selects a combination of users having the largest capacity.
Channel vector measurement means 811 measures a channel vector in up-link by using a pilot signal of each user (mainly the pilot signal for sounding periodically transmitted during non-transmission of data). User combination MIMO capacity calculating means 812 calculates the MIMO capacity of a combination of all users. The combination number of all users is Nall=NuCM, and taking a channel matrix in a combination k (k=1, 2, . . . , Nall) of users as H(k), the MIMO capacity may be expressed by the following equation.
                                                                        C                ⁡                                  (                                      H                                          (                      k                      )                                                        )                                            =                            ⁢                                                log                  2                                ⁢                                  det                  ⁡                                      (                                          I                      +                                                                                                    P                            s                                                                                P                            n                                                                          ⁢                                                  H                                                      (                            k                            )                                                                          ⁢                                                  H                                                                                    (                              k                              )                                                        ⁢                            H                                                                                                                )                                                                                                                          ≈                            ⁢                                                M                  ⁢                                                                          ⁢                                      log                    2                                    ⁢                                                            P                      s                                                              P                      n                                                                      +                                                      log                    2                                    ⁢                                      det                    ⁡                                          (                                                                        H                                                      (                            k                            )                                                                          ⁢                                                  H                                                                                    (                              k                              )                                                        ⁢                            H                                                                                              )                                                                                                                              (        5        )            Where, Ps is transmission power per user, and Pn is noise power. User selection means 113 selects combination kopt of users having the largest MIMO capacity C(H(k)).
                              k          opt                =                  arg          ⁢                                          ⁢                                    max              ⁢                                                                    k                    ⁢                      C            ⁡                          (                              H                                  (                  k                  )                                            )                                                          (        6        )            
The user selection method by the whole search method has to calculate MIMO capacity C(H(k)) in Nall ways, and the amount of calculation becomes massive as number Nu of all users and number M of simultaneous transmission users (=the number of receiving antennas) are increased.
FIG. 6 shows a configuration of a device for user selection by a Gram-Schmidt (GS) orthogonalization method in which the channel orthogonality is considered.
The user selection by the GS orthogonalization method performed in the device shown in FIG. 6 includes channel vector measurement means 911, projection channel power calculating means 914, user selection means 915, completion determination means 916, and projection channel vector updating means 917, and is a method for selecting a user having a large power of a projection channel vector of each user acquired by using GS orthogonalization for each MIMO multiplexed layer by each TTI, and the method is disclosed in, for example, “Multiuser diversity for a dirty paper approach” by Z. Tu and R. S. Blum, IEEE Commun. Lett., vol. 7, no. 8, pp. 370 to 372, August 2003 (Non-Patent Document 1). QR decomposition (H(k)=Q(k)R(k)) is applied to channel matrix H(k) in equation (5), and combination kopt of users having the largest MIMO capacity may be expressed by the following equation.
                              k          opt                =                              arg            ⁢                                                  ⁢                                          max                k                            ⁢                                                                                      det                    ⁡                                          (                                              R                                                  (                          k                          )                                                                    )                                                                                        2                                              =                      arg            ⁢                                                  ⁢                                          max                k                            ⁢                                                ∏                                      m                    =                    1                                    M                                ⁢                                  r                                      m                    ⁢                                                                                  ⁢                    m                                                                              (                      k                      )                                        2                                                                                                          (        7        )            
Where, rmm(k) represents a diagonal element of R(k), and the MIMO capacity can be maximized when a user is selected so that the square product of rmm(k) is maximized. To realize this quasi-optimally, the GS orthogonalization is used. The GS orthogonalization corresponds to a process in which the QR decomposition is performed while selecting a user in sequence so that rmm(k) becomes largest. FIG. 7 shows processes of the GS orthogonalization and the QR decomposition. The process of the GS orthogonalization ends after an M-th user is selected from among the Nu users. Further, FIG. 8 shows a concept of the GS orthogonalization. In the GS orthogonalization, while an orthogonal axis corresponding to a user which can project channel vector hj of a user j as largely as possible is selected in sequence, a process for updating projection channel vector hj(n) of hj projected onto complementary space Q(n)⊥ of orthonormal system Q(n) composed of n orthogonal axes already selected is repeated.
Channel vector measurement means 911 measures a channel vector in up-link by using a pilot signal of each user. Projection channel power calculating means 914 calculates power of projection channel vector hj(n−1) of a user j updated by using the GS orthogonalization. Here, when n=1, hj(0)=hj. User selection means 915 selects a user having the largest projection channel power of each user. User Jn to be selected in an n-th MIMO multiplexed layer is selected by using the following equation.
                              J          n                =                  arg          ⁢                                          ⁢                                    max                              j                ∈                A                                      ⁢                                                                            h                  j                                      (                                          n                      -                      1                                        )                                                                              2                                                          (        8        )            
Further, selected user Jn is deleted from a set of users to be selected in a next MIMO multiplexed layer. Completion determination means 916 outputs user selection information after completing the selection of users by number M of simultaneous transmission users, and moves to a process for a next user selection when n<M. Projection channel vector updating means 917 updates projection channel vector hj(n) of the user j onto complementary space Q(n)⊥ of orthonormal system Q(n) corresponding to the user already selected by using the GS orthogonalization.
                              h          j                      (            n            )                          =                              h            j                          (                              n                -                1                            )                                -                                                    (                                                      h                                          J                      n                                                                                      (                                                  n                          -                          1                                                )                                            ⁢                      H                                                        ⁢                                      h                    j                                          (                                              n                        -                        1                                            )                                                                      )                            ⁢                              h                                  J                  n                                                  (                                      n                    -                    1                                    )                                                                                                                      h                                      J                    n                                                        (                                          n                      -                      1                                        )                                                                              2                                                          (        9        )            
As described above, the user selection methods based on the received SINR (the maximum CIR method and the PF method) have a problem that the multiuser diversity effect cannot be satisfactorily provided because the channel orthogonality of a pair of users is not considered.
The user selection method by the whole search method can provide a satisfactory multiuser diversity effect, but has a problem that the amount of calculation becomes massive as the number of all users and the number of simultaneous transmission users are increased.
The user selection method by the GS orthogonalization method has a problem that interference power is not considered because of a signal power criterion based on the projection channel power of each user. It may be conceived that the interference power of each user differs in measurement at a frequency and a different time.
Further, the user selection method by the GS orthogonalization method is a user selection method according to the maximum capacity criterion, that is, the method is a channel aware (CA) method (the CA method may correspond to the conventional, maximum CIR method, which is here called “CA method” because the channel orthogonality in addition to CIR is considered in MU-MIMO), and is not disclosed about application to the user selection method by the PF method or a user selection method in which priorities (a delay request, a retransmission request, and the like) except the channel quality are considered.